Watercraft are often propelled by an outboard motor. These motors have a water propulsion device, such as a propeller, which is driven by an output shaft of an internal combustion engine. The engine is typically mounted in a cowling of the motor.
The motor includes a lubricating system for providing lubricant to the engine. These systems are well known in the art, and arranged to provide lubricant from a supply to one or more galleries, bearings and the like of the engine for lubricating the various parts thereof.
When the lubricant in such a system is too cool or too hot, the effect of the lubricant is less than optimal. Typically, the optimum operating temperature range for the lubricant is in the 60.degree. C. to 80.degree. C. temperature range. When the temperature of the lubricant is less than this range, it is difficult to pump and flows less freely through the lubricating system and engine. On the other hand, when the temperature of the lubricant exceeds this range, the lubricant thins and becomes less effective in providing a protective lubricant film on the moving parts of the engine.
The problem of overheating the lubricant is especially a problem in outboard motor applications since the engine is positioned in an enclosed cowling, trapping engine heat. Some lubricant systems are provided with a lubricant cooling system to prevent the lubricant from becoming overheated. The cooling system of an outboard motor is generally arranged such that water from the body of water in which the motor is being operated is drawn by a pump and delivered through one or more cooling jackets associated with the engine and then discharged back to the body of water. In one lubricating cooling arrangement, lubricant is delivered through a delivery line which passes through a heat exchanger through which coolant from the cooling system of the motor also passes. In this arrangement, the fixed flow of coolant passing through the heat exchanger has the tendency of over-cooling the lubricant when the engine speed/temperature is low. As one attempt to correct this problem, the coolant flow rate through the heat exchanger may be fixed at a low rate which does not over-cool the lubricant. This arrangement has the problem of providing insufficient cooling to the lubricant when the engine temperature increases.
The problems associated with maintaining the lubricant temperature are aggravated in the outboard motor setting since the coolant may comprise extremely cold water from the ocean or a lake. In that situation, the coolant temperature is so low that when the lubricant temperature is low, the coolant lowers the lubricant temperature to an unacceptably low level.